On a Tecnam sierra equipped with a rotax 914 (2,5 years old 60 hrs total)there was this fault.
Perfect running until 100% throttle position but when advancing to 115% there was intermittent power (huge fluctuation of exhaust noise and thrust). When returning to 100% engine return normal. it happen on first day takeoff and was very very scary.
Because defect is only on maximum boost first check is turbo wastgate/TCU operation but it was ok.
Second check was for oil contamination in airbox/carbs (leaking turbo oil feed check valve is a common issue on 914 and this particular engine has a history of little usage and long resting). But in this case oil presence is minimal and rotax mechanics says fisiologic for this type of engine.
after long thoughts we realized that due to already said little usage of plane (less than 10 hours in last year) gasoline was VERY AGED.
After draining and refueling with fresh mogas engine run ok at all power.
But WHY problem was only with throttle after detent position (115%). We suspects that because on 914 there was an automatic fuel enrichment at maximum power (>108% commanded by control unit ) old gasoline was less volatile than fresh one and resultant mixture was way too rich to properly ignite.
Moreover it was first flight in winter cold condition and this helped to aggravate the enrichment (during winter gasoline is formuled to be more volatile).
Someone has incurred in this subtle problem? or have a different explanation ?
thanks
Nicola di Biase
Perfect running until 100% throttle position but when advancing to 115% there was intermittent power (huge fluctuation of exhaust noise and thrust). When returning to 100% engine return normal. it happen on first day takeoff and was very very scary.
Because defect is only on maximum boost first check is turbo wastgate/TCU operation but it was ok.
Second check was for oil contamination in airbox/carbs (leaking turbo oil feed check valve is a common issue on 914 and this particular engine has a history of little usage and long resting). But in this case oil presence is minimal and rotax mechanics says fisiologic for this type of engine.
after long thoughts we realized that due to already said little usage of plane (less than 10 hours in last year) gasoline was VERY AGED.
After draining and refueling with fresh mogas engine run ok at all power.
But WHY problem was only with throttle after detent position (115%). We suspects that because on 914 there was an automatic fuel enrichment at maximum power (>108% commanded by control unit ) old gasoline was less volatile than fresh one and resultant mixture was way too rich to properly ignite.
Moreover it was first flight in winter cold condition and this helped to aggravate the enrichment (during winter gasoline is formuled to be more volatile).
Someone has incurred in this subtle problem? or have a different explanation ?
thanks
Nicola di Biase
